In the art of making tires, it is desirable to employ rubber vulcanizates that demonstrate reduced hysteresis loss, i.e., less loss of mechanical energy to heat. Hysteresis loss is often attributed to polymer free ends within the cross-linked rubber network, as well as the disassociation of filler agglomerates.
Functionalized polymers have been employed to reduce hysteresis loss. The functional group of the functionalized polymer is believed to interact with a filler particle and thereby reduces the number of polymer free ends. Also, the interaction between the functional group and the filler particles reduces filler agglomeration, which thereby reduces hysteretic losses attributable to the disassociation of filler agglomerates.
One particularly useful functional group is tributyltin. Rubbery polymers containing a tributyltin functionality have produced rubber vulcanizates that demonstrate reduced hysteresis loss. These rubbery polymers have been prepared by anionic polymerization techniques whereby tin-lithio initiators, e.g., tributyltin lithium, have been employed as initiators.
Further reduction in hysteresis loss has been observed where the rubber polymers employed in the manufacture of tires contain functionalities at both the head and tail of the polymer. For example, polybutadiene and poly(styrene-co-butadiene) have been prepared by initiating their polymerization with a tin-lithio initiator, e.g., tributyltin lithium, and terminating the polymerization with alkyltin chlorides, which impart a tin functionality at the end of the polymer chain.
While polymers that have functionalities at both their head and tail have demonstrated the ability to provide filler-reinforced vulcanizates with advantageous hysteresis properties, the ability to process these polymers is reduced as compared to non-functional polymers. Namely, the ability to adequately mix filler particles into the rubber compound has proven to be problematic because it requires greater mixing energy and mixing time.
Because polymers that provide filler-reinforced vulcanizates with reduced hysteresis loss are important in the manufacture of tires, there is a need to overcome problems associated with prior art polymers.